Closed cycle gas cryogenically cooled radiation detector

ABSTRACT

A radiation detector having an evacuated envelope, a radiation detector on a cold finger support in the evacuated space, a closed cycle gas cooling system to cool the cold finger to provide cryogenic operation of the radiation detector, and a getter in the evacuated space to maintain an evacuated condition. The evacuated envelope includes a radiation window. The radiation detector is preferably an X-ray detector employed in an energy dispersive spectrometry system. The evacuated space is preferably held at a pressure of less than about 1 mTorr to achieve molecular flow of remaining gas molecules, minimizing parasitic heat input. The closed cycle gas cooling system employs compressed refrigerant, which is precooled in a counterflow heat exchanger and allowed to expand in proximity to the cold finger, thus absorbing heat and maintaining cryogenic temperatures. A getter material, preferably activated carbon, is provided to absorb gasses and maintain the low pressure during operation. A vibration effect attenuation system is provided to reduce effect of cooler induced reduction in detector resolution.

FIELD OF THE INVENTION

The present invention relates to the field of cryogenic radiationdetectors, and more particularly to mechanically cooled cryogenicradiation detectors operating under vacuum conditions.

BACKGROUND OF THE INVENTION

Radiation detectors, which may be X-ray detectors, infrared radiationdetectors or other types, commonly have an evacuated envelope with avacuum space, at least one radiation detector element being mounted inthe vacuum space. These devices include scanning electron microscopes(SEM), X-ray spectrometers, infrared spectrometers, and other knowndevices. For optimal operation, with low electronic noise and highsensitivity, these detectors are operated cryogenically. Therefore, thedetector is provided with a cooling device. In order to reduce parasiticheat input to the system, the detector and cooling system are generallyinsulated from the environment in a Dewar, having a vacuum space withina sealed envelope or chamber. A cooling element is provided in thechamber and serves to cool the detector element during operation of thedetector. The vacuum minimized heat conduction through a gas-filledspace, and the surfaces are formed with low heat radiation emissivityconstruction. In general, the detector may be allowed to return toambient temperature when not in operation.

A member, in thermal communication with the cooling device, protrudesinto the chamber and supports the detector. As is known, the member isactively cooled by the Joule-Thomson effect (gas expansion), liquidnitrogen (or other gas), Stirling cycle cooling, Peltier junctions, orby other known means. The cooled protruding member is often termed "thecold finger" of the detector. The cold finger is also thermally coupledto a detector to be cooled, and generally acts as a mechanical supportas well.

In order to achieve the desired cryogenic temperatures, around 87K atthe junction of the detector holder and cold finger for lithium driftedsilicon X-ray detectors, and low vibration, the Dewar is typicallyfilled with liquid nitrogen, which exists at 77K in the liquid state atstandard pressure, which is allowed to vaporize, providing cooling ofthe cold finger in the cold-finger assembly. The radiation detector isthermally conductively attached to an end of the cold finger oppositethe cryogenically cooled end. The finger is thus insulated from ambientatmosphere and contained within a housing. The vaporization of liquidnitrogen in a Dewar to achieve cryogenic temperatures is an inherentlylow vibration-producing action, and therefore liquid nitrogen istypically employed to cool vibration sensitive instruments. Liquidnitrogen is expensive, requires frequent refilling of an internal supplyin the detector, and is often delivered in bulk from off-site, requiringscheduled service. Further, the handling of liquid nitrogen may behazardous and undesirable in certain environments, such as clean roomsand remote locations.

It is known that a significant cause of detector failure is the gradualdegradation of the vacuum in the evacuated space due to, e.g., internalout-gassing of the various component parts of the detector exposed tothe vacuum and leakage from outside the evacuated envelope. In order toreduce outgassing of the component parts of the detector in the vacuumspace during the service life of the detector, these component parts aregenerally prebaked in known manner in vacuo before assembly, and ageneral bakeout of the assembly is also carried out before mounting thedetector electronic element(s). The excessive outgassing which generallyoccurs in X-ray and infrared detectors may be due to the fact that thegases cannot be driven out by baking the whole device during vacuumpumping (in the way which is usual for other vacuum devices) becauseX-ray or infrared detector elements are damaged at high temperatures.

This degeneration in the vacuum eventually leads to the situation inwhich a cooling element is no longer able (at least in an efficientmanner or sufficiently fast) to cool the detector element to the desiredtemperature for efficient detection of the radiation. Thus, the detectorlifetime is curtailed, especially as only limited cooling power isavailable to cool the detector. Furthermore, the outgassing into thevacuum space provides a significant thermal transfer path between thecold finger and the outside of the detector, when the pressure exceeds1₋₋ 10⁻³ Torr. In mechanical cooling systems, the thermal capacity dropsas the temperature differential increases, so that with an insufficientinitial vacuum, necessary operating temperatures may never be reached.

The cooling system is generally not provided with an extraordinarilylarge cooling capacity because this may introduce increased vibration,leads to inefficiency and increased size, and may be difficult tocontrol. In operation, the desired vacuum condition in the housingreduces the heat transfer from outside the vacuum space, thus limitingthe amount of heat which must be removed through the cold finger. As thevacuum deteriorates, heat transfer from outside the space increases,imposing a greater heat load on the cryogenic system. Thus, the detectorlifetime may be curtailed.

U.S. Pat. No. 3,851,173 describes one example of an infrared detectorcomprising an envelope, a cold finger in the envelope and at least onedetector element mounted on an end of the cold finger so as to be cooledby the cold finger in operation of the detector. The envelope has anouter wall extending around the cold finger and an infrared-transmissivewindow facing the detector-element end of the cold finger. A space ispresent between the cold finger and the window and outer wall of theenvelope, and a chemically-active getter is present in said space togetter gases from the space. The envelope of U.S. Pat. No. 3,851,173 isa vacuum Dewar having a vacuum space between inner and outer walls ofthe Dewar envelope. The inner wall defines an inner chamberaccommodating a cryogenic cooling element which serves to cool the innerwall end and hence the detector element thereon, during operation of thedetector. The cooled inner wall forms the cold finger of the detector. Amajor cause of infrared detector failure in designs of this type is thegradual degeneration of the vacuum in the space between the inner andouter walls due to internal out-gassing of the various component partsof the detector exposed to the vacuum. In the detector of U.S. Pat. No.3,851,173 a non-evaporable chemically-active SAES getter unit is mountedin the outer wall to maintain a good vacuum in the space between theouter wall and the cold finger.

Another example of an infra-red radiation detector incorporating agetter to maintain a vacuum in a Dewar is described in U.S. Pat. No.3,786,269. Its detector element array is cooled by a Stirling cyclerefrigerator at approximately 50° K. In this particular detector aseries of chemically active getters are mounted around the outerperimeter of the outer wall and protrude through into the vacuum spacebetween the outer wall and the cold finger. In order to gettersufficiently large quantities of gas, such chemically active gettersneed to have a large surface area and are bulky; this can present adimensional size problem in the spacing of the inner and outer wallsand/or the shape of the outer wall.

Such chemical getters as employed in U.S. Pat. Nos. 3,851,173 and3,786,269 are activated by being taken to a high temperature (forexample 900° to 1,000° C.) during or after evacuating and sealing theDewar envelope. This is normally achieved with an electrical heatingelement embedded in the getter material formed as a unit with electricalconnection leads (not specifically shown in U.S. Pat. No. 3,851,173)passing through vacuum-tight seals in the Dewar. U.S. Pat. No. 4,206,354shows an example of such a Dewar getter with its connection leads. Forthis reason the getter is mounted in the outer envelope wall withexternal electrical connections, and a large spacing is required betweenthis type of getter and the detector element which could otherwise bedamaged by the very high temperature. These factors lead to an increasedsize for the Dewar envelope and even the adoption of unconventionalDewar envelope outlines.

U.S. Pat. No. 5,177,364, incorporated herein by reference, discloses acryogenic infrared detector system having a chemically-active getterpresent in an evacuated space to getter gases from the space. The gettercomprises a porous layer of chemically-active getter material depositedon an inside surface area of the outer wall at a location separated fromboth the cold finger and the window. United Kingdom patent applicationGB-A-2,179,785 describes a pumping tubulation getter device for anelectron discharge device such as a ring laser gyroscope. GB-A-2,179,785describes the provision of the getter as anelectrophoretically-deposited layer of porous sintered non-evaporablegetter material selectively deposited on the internal surface of thepump tube through which the device is evacuated. Getter-free zones arepresent at each end on the internal surface of the tube. The getter isactivated by HF induction heating of the tube. By locating the getterwithin the pump tube, space problems which otherwise arise in trying toaccommodate a getter unit within the device chamber are avoided.

U.S. Pat. No. 5,235,817, incorporated herein by reference, discloses aradiation detector cryogenic cooling apparatus having a plurality ofnested space thermally conductive elongated members having first andsecond ends. These nested tubes reduce thermal radiation parasitic inputto the detector and cold finger, allowing higher efficiency and lowerdetector temperatures to be achieved. U.S. Pat. No. 5,274,237 relates toa cryogenically cooled radiation detector having means for preventingicing.

The use of molecular-sorbent porous bodies as getters is known. See,e.g., "Zeolite and Molecular Sieves" by D. W. Breck, John Wiley andSons, Inc., New York, London and Sydney (1974) for a general backgrounddescription of such porous molecular sorbents. It is known to usemolecular-sieve getters in the form of loose beads or loose pelletsretained behind a screen or in a cage. See, GB-A-921,273 which relatesto liquefied gas storage containers and GB-A-1,192,897 which relates tocircuit breakers. Molecular-sorbent getters are known to have increasedsorption efficiency at cryogenic temperatures, unlike chemically activegetters, which have increased sorption affinity at higher temperatures,e.g., about 300° K. and above.

Activated carbon molecular sorbent getter is known. See, e.g., Norit®activated carbon, Product Information Bulletin No. 206 (Rev. 1-91), fromAmerican Norit Company, Inc., incorporated herein by reference.

Activated carbon may be provided as granules or in shaped forms.Preshaped forms are provided either by pyrolizing a preformed organicmaterial, which may be a polymer or organic mass, or by providing abinder for previously activated carbon powder. During pyrolysis, aproperly selected organic material will undergo a predictabledimensional alteration; therefore a preformed organic material may beprovided to pyrolize into the desired shape.

The getter of U.S. Pat No. 4,474,036 is, for example, a zeolite orsynthetic zeolite material. The getter may also be a molecular sorbentmaterial formed in situ, minimizing the need for adhesives, such assilica gel. This material is formed into a shape which conforms to acooled portion of the vacuum space, such as around the cold finger, andis preferably not a loose granulate, because thermal transfer throughsuch a material is retarded, and firm, thermally transmissive contactwith the cooled portion of the Dewar cannot be assured. Therefore, thegetter is generally provided as a formed element in substantial thermalcontact with a cooled portion of the Dewar, such as by a low-outgassingepoxy. The epoxy may be filled with a thermally conductive material,such as silver powder. The getter is generally very fragile so that itis preferably supported by other structures. It is considered desirablein this application to effect reduction of the pressure in the evacuatedspace to less than about 1₋₋ 10⁻³ Torr in less than about 30 secondsafter the time the getter is cooled.

Typically, the synthetic zeolite body of U.S. Pat. No. 4,474,036 iscomposed of particles having a width of at most a few micrometers andwith somewhat irregular inter-particle voids also in the body. The poresof the porous zeolite particles forming the body have a width comparableto molecule sizes (up to approximately 0.5 nm) of gases in the vacuumspace and were formed by driving off the water of crystallization of thezeolite material before molding the zeolite particles together in anannular shape to form the body; the heating required to effect thisdehydration is thus performed before mounting the getter in the Dewarenvelope. The resulting molecular-size pores permeate the zeoliteparticles to give an extremely large internal surface area, as a resultof which the cooled body can absorb a large volume of gas by adsorptionon the inner surfaces of the pores.

Since the cooling element of U.S. Pat. No. 4,474,036 is provided to coolto only a moderate cryogenic temperature, the good large-area thermalcontact between the inner major surface of the getter body and thesurface of the Dewar is particularly important in efficiently coolingthe molecular-sieve body, i.e., the molecular sorbent getter, to obtaina high sorption efficiency. An annular configuration for both the getterbody and the cooled surface also minimizes the amount of epoxy adhesivenecessary to secure the getter body to the surface; this is importantsince a large amount of epoxy can increase out-gassing into the vacuumspace. In a particular example the epoxy film may be typically 100micrometers thick.

Molecular-sorbent getters do not require activation heating to very hightemperatures after mounting in a vacuum space, so that the getter can bemounted in the proximity of the detector element so as to obtain maximumcooling of the molecular-sorbent porous body. A most efficient coolingof the shaped molecular-sorbent porous getter body can be achieved whenthe body is mounted around the inner wall of the detector Dewar in avicinity where the inner wall is directly cooled by the cooling element.The shaped getter body or bodies of molecular-sorbent porous materialmay be bonded to an outer surface of the inner wall, and/or any othercooled surface associated with the inner wall. Thus, an annularradiation shield may be mounted at the end of the inner wall around thedetector element, and the cooled surface to which at least one saidshaped getter body is secured may be an outer surface of the radiationshield.

These molecular sorbent getters may be employed to reduce the effect ofinternal out-gassing in infrared detectors. Thus, it is known to provideat least one molecular-sorbent getter in a vacuum space of a housing forgettering gas molecules from this space. According to U.S. Pat No.4,474,036, incorporated herein by reference, having an infra-redradiation detector mounted in an evacuated space with an infraredtransmissive window, and cooled by a cold finger. U.S. Pat. No.4,474,036 discloses the use of Joule-Thomson effect cooling for thecryogenic infrared detector system, e.g., by allowing a gas, such as dryair, nitrogen or argon, to expand in an area of lower pressure,absorbing heat. See also EP-A-0,006,297. In general, infrared detectorsof the type employed in U.S. Pat. No. 4,474,036 are not especiallysensitive to vibration and therefore the intrinsic vibrations from aJoule-Thomson cooling system did not require redress.

Cryogenic cooling apparatuses for cooling other types of radiationdetectors to cryogenic temperatures are also known. For example, aradiation detector is employed with an electron microscope for detectingX-rays incident on a specimen being spectroscopically examined. SuchX-ray detectors do not require an optically transparent window. Thespecimen is placed within the microscope and receives incident electronbombardment from the microscope. Scattered radiation from the specimenis then detected by a cryogenically cooled detector which converts theradiation to an electrical signal in a known manner for spectroscopicanalysis. The detector is mounted on an elongated structure referred toin the art as a cold finger, extending to a position where it is desiredto detect X-rays. The finger is cantilevered to a support so as to beplaced within the region of the electron microscope adjacent to thespecimen. The interior of the microscope and the region surrounding thecold finger are within an evacuated chamber. Cooling of the detector isaccomplished by the finger which is thermally conductively connected toa source of cryogenic cooling, for example, a Dewar containing liquidnitrogen.

U.S. Pat. No. 5,337,572, incorporated herein by reference, relates to aclosed cycle cryogenic refrigerator. This system employs a ternarymixture of gasses to allow a single stage compressor to achieve coolingtemperatures of between about 65 and 150K, when used with aJoule-Thomson cryostat. U.S. Pat. No. 5,313,801 relates to a throttlevalve for a cryostat, allowing automatic internal temperature regulationwith a minimum of moving parts.

A Joule-Thomson cryostat operates by allowing an isenthalpic expansionof a medium, thereby cooling the medium. Therefore, a compressedrefrigerant is provided to a flow restricting orifice. An expansionchamber is provided after the orifice, having a greater cross sectionalarea than the orifice. In general, a turbulent process occurs in theexpansion chamber, producing vibration. Such a cryostat may operate witha liquified refrigerant, especially to obtain cryogenic temperatures.Such liquids generally expand supersonically and turbulently at the flowrestricting orifice.

U.S. Pat. No. 4,910,399 discloses an electron microscope with an X-raydetector in an arrangement as described above. U.S. Pat. No. 3,864,570also disclose an X-ray detector for use with an electron beam producingdevice disclosing a cold finger structure. British Pat. No. GB 2,192,091discloses a still further embodiment of an electron microscope andcryogenically cooled X-ray detector system.

Typically in these kinds of systems, it is known to reduce heat input tothe cold finger mounting the X-ray detector by using low emissivity warmsurfaces and by wrapping the cold finger with low emissivity aluminizedMylar®. The heat to be extracted from the system comes from foursources. First, the X-ray detector system, including the detectorcrystal and electronics, generates an amount of heat in operation.Second, gas molecules in the evacuated space conduct heat to thedetector and cold finger. Third, mechanical support structures bridgingthe cooled elements, i.e., the cryogenic cooler, the cold finger, andthe detector assembly, and warm structures, i.e., the outer housing,conduct heat. Fourth, energy is radiated to the detector and coldfinger, both heat energy from warm surfaces and energy from theoperation of the apparatus, a portion of which is absorbed. At steadystate temperature, the cold finger conducts the heat input along itslength to the cooling system, and there is a temperature gradientbetween the radiation detector at the end of the cold finger and theheat sink of the cooling system. Care must be taken to minimize thisgradient for acceptable performance. Additionally, the Mylar® and otherorganic compounds used in the insulation system present in the evacuatedchamber in which the cold finger is secured may evolve contaminantsundesirable when used in a ultra-high vacuum (UHV) environment.

In an electron microscope, a cavity of the electron microscope receivesa specimen being examined by an electron beam produced by themicroscope. The beam, typically less that 1 micron diameter, is incidenton the specimen, producing X-rays which are then radiated from thespecimen. The detector is placed within the microscope cavity adjacentto the specimen and detects the radiation emanating from the specimen.The detector, which includes a semiconductor detector for converting theX-ray signals to electrical impulses and a field effect transistor (FET)for sensing and amplifying the detected signal, produces an electricalsignal which is passed to an external electronic circuit for analysis.

For example, one such X-ray detector system is disclosed in U.S. Pat.No. 4,931,650. In this environment the known detector comprises asemiconductor mounted at the end of the cold finger or probe introducedinto the microscope close to the specimen. The cold finger is surroundedby an envelope and a vacuum is maintained between the finger and theenvelope. The cavity in the microscope receiving the cold finger is alsoheld at a vacuum. According to U.S. Pat. No. 4,931,650, a problem withX-ray detection in this apparatus is that it is sensitive tocontamination and, especially, ice buildup. Moisture tends to accumulateon the detector, decreasing its effectiveness. This moisture may beremoved and the performance of the detector improved by a warming-upprocedure. Generally, prior art systems require that the system bedisconnected for a period of time usually every few days or, in somecases, hours, so as to warm up the system and remove the accumulatedmoisture. The warming-up procedure involves pumping the detector tomaintain a vacuum while removing water vapor as it evaporates. Such aprocedure is used only as part of a major overhaul involving the returnof the detector to the manufacturer. For windowless detectors, awarming-up procedure may involve using the vacuum pumping system of theelectron microscope, which must maintain a vacuum during operation. In awindowed device for spectroscopy type examination, the cold finger ispermanently maintained in its own evacuated housing. With liquidnitrogen cooling and sufficient getter material, an acceptable vacuummay be maintained for years.

In U.S. Pat. No. 4,886,240 a non-evacuated Dewar is disclosed whichemploys a molecular sieve that serves to absorb gases in the Dewar whencooled, for operation of a detector and to prevent liquid formation ontothe detector. A desiccant also may be used to absorb moisture. Themolecular sieve is employed for removing gases from the area adjacent tothe detector when operating. Fluid contained within the cold fingerexpands, thereby absorbing thermal energy. The Dewar housing is backfilled with inert gas such as nitrogen. This gas is at one atmosphere,e.g., at atmospheric pressure.

Known systems require the use of liquid nitrogen to achieve sufficientlylow temperatures for high performance operation, e.g., cryogenictemperatures with low vibration for high resolution. Liquid nitrogen,however, is undesirable in certain applications, such as semiconductorclean rooms and remote lab sites. Peltier junction (thermoelectric)coolers have great difficulty in achieving sufficiently low temperaturesfor high performance operation. See, U.S. Pat. No. 5,075,555, whichrelates to a Peltier cooled lithium drifted X-ray spectrometer.Mechanical cooling systems, such as Stirling cycle refrigerators requirecomplex mechanisms near the cold finger and may introduce vibrations.

Systems are known which attempt to actively damp vibrations. See Garbaet al., "Piezoelectric Actuators on a Cold Finger", Technical SupportPackage, NASA Tech Brief 19(1) item 277, JPL New Technology ReportNPO-19090, incorporated herein by reference. Such systems require atleast one actuator for each axis of compensation and either a knownpredetermined vibration pattern or sensors to determine the vibration tobe damped.

U.S. Pat. No. 5,225,677 relates to a protective coating for an X-raydetector. U.S. Pat. No. 5,268,578 relates to a specially shaped X-raydetector.

SUMMARY OF THE INVENTION

The present invention relates to a cryogenic radiation detection systemhaving a radiation detector in an evacuated chamber in which thecryogenic condition is maintained by a closed cycle gas cryogeniccooling system. The detection system preferably includes an X-raydetector or other radiation detector, mounted on a cold finger in avacuum. The cryogenic radiation detection system may be employed inenergy dispersive radiation spectroscopy, e.g., X-ray spectrometry,elemental analysis in electron microscopy, X-ray fluorescence analysisand nuclear spectroscopic analysis.

In order to achieve the necessary cryogenic operating temperatures forthe detecting unit, while employing an efficient cooling system, it isnecessary to maintain a vacuum level of at most 1₋₋ 10⁻³ Torr inside thedetecting unit. This vacuum level reduces the parasitic heat input tothe system by substantially eliminating conduction through any gas inthe detecting system cryostat. Available cryocoolers without this typeof insulation will not reach operational temperatures at the detectorcryostat necessary for low noise operation.

The closed cycle gas cryogenic system does not require a source ofexpendable liquid nitrogen, and allows use of a remotely locatedcompressor/condenser unit. The compressor is preferably air-cooled andelectrically operated, and supplies high pressure refrigerant to thedetecting unit. The compressor/condenser is linked to the detecting unitby refrigerant supply and return lines. The compressed refrigerant fromthe supply line is at approximately room temperature. The compressedrefrigerant is fed to a cryocooler in the detecting unit, where it isprecooled by counterflow heat exchange from returning expandedrefrigerant. An expansion chamber is provided in the cryocooler wherethe precooled compressed refrigerant is allowed to expand, where itabsorbs heat and cools the surrounding mass, which is in thermalcommunication with the cold finger of the detector, which in turn coolsthe detector.

The cryogenic temperatures in the detecting unit are achieved by using aclosed cycle, throttle valve regulated refrigerator. This device hasbasically no moving mechanical parts in proximity to the detector, andhas low vibration characteristics at the detector end, when properlydamped. The preferred cooler system is capable of cooling the detectingunit cold finger to approximately 100K at the detector crystal end, witha temperature stability of ±0.5K. A temperature responsive throttlevalve is provided in the cryocooler to control the flow of compressedrefrigerant into the expansion space. This throttle valve isself-regulating, controlling the refrigerant flow by the thermalexpansion and contraction of an occlusive member acting as a needlevalve. The refrigerant returning from the expansion space is provided ina conduit which flows antiparallel to the compressed gas entering thecryocooler unit. This exhaust refrigerant is cool, because therefrigerant in the expansion space is at about 80K, and thereforeprecools the entering compressed refrigerant to a low temperature. Underthese temperature and pressure conditions, the compressed refrigerantnear the throttle valve is mostly liquified. The counterflow heatexchanger has sufficient length so that the exhaust gas exiting from thecounterflow heat exchanger is at about ambient temperature, therebyproviding high efficiency. The preferred cryocooler has a thermalcapacity of about 2 Watts when the thermally conductive member is about82K.

The refrigerant may also be regulated in other manners, such as apiezoelectric valve, for example having a pulse modulated apertureleading to the expansion space. This allows more control over the valveoperation, and may reduce turbulence-induced vibration at the detectorwhen the valve is closed. Thus, readings under some circumstances may betaken in synchronization with low vibration intervals of operation. Ofcourse, a synchronized reading method may be slower than a continuousreading method, and may best employed where reading throughput need notbe maximized, such as where other time-constants of system inoperabilityare significantly larger than the valve modulation rate. A manualcontrol valve may also be employed.

The detector is preferably a solid state semiconductor elementinterfaced with a field effect transistor (FET) amplifier circuit withinthe cryostat. At the detector crystal end of the housing assembly thereis a thin window to allow X-rays to enter the detecting unit, whilemaintaining a vacuum. The thin window may be formed of, e.g., berylliumor thin polymer type materials. The solid state detector is maintainedat cryogenic temperatures in order to reduce the electronic noise of thedetector crystal and the FET.

The detector crystal and FET are mounted close together to one end thecold finger, which is a copper rod. The detector mount itself ispreferably formed of aluminum. The other end of the cold finger, whichis preferably formed of copper, is thermally connected to the cryocoolerrefrigeration unit. The cold finger is mechanically mounted within thecryostat by a temperature insulating support member affixed within thehousing. A system is provided for the purpose of vibrationally isolatingthe cold finger from the cryocooler so that vibrations in and from thecryocooler are attenuated, thereby increasing the energy resolution ofthe detector.

When employing a lithium drifted silicon X-ray sensor, a temperature ofabout 100K to 110K at the detector crystal is desired. According to thepresent design, an internal mass damper has an operating temperature ofabout 82K, with a gradient along the cold finger and detector holder.Other types of detectors may have different optimum temperatures. Forexample, germanium detectors, which may detect X-rays and/or gamma rays,generally require a lower temperature, e.g., about 95K, for effectiveand efficient operation. In this case, a closed circuit cryocoolersystem is provided which produces lower temperatures, thus allowingreduced temperatures at the detector. Improved insulation techniques toreduce parasitic heat load may also reduce temperatures at the detector.The cryocooler may be operated with a refrigerant composition effectivefor cooling to temperatures lower than those achieved through the use ofliquid nitrogen, which exists in a liquid state at standard pressure at77K. For example, a cryocooler system similar to the system disclosed inU.S. Pat. No. 5,337,572, incorporated herein by reference, may be usedto achieve minimum temperatures of less than 70K.

In order to reduce the thermally radiated heat input to the detector andcold finger, a cooled concentric tube about the cold finger may beemployed to shunt heat radiated from the housing from the cold finger,in the manner of U.S. Pat. No. 5,235,817, incorporated herein byreference. This significantly reduces the parasitic heat load on thecold finger, reducing the temperatures achievable at the detector. Thus,the a cryocooler system according to the present invention allows thedetector to operate at temperatures lower than those achieved with aliquid nitrogen Dewar. When a large amount of heat is input into aliquid nitrogen Dewar, vibration due to liquid nitrogen phase changebecomes significant, and may couple to the electron microscope, reducingspatial resolution.

The supply and return lines link the compressor/condenser to thecryocooler. These lines are preferably formed of malleable copper tube,and may be of arbitrary length. Self sealing Aeroquip refrigerantcouplings are provided to allow for easy connection and disconnection ofthe cryocooler from the compressor/condenser. The compressorcontinuously draws low-pressure refrigerant from the cooling systemreturn line. The refrigerant is compressed, cooled and filtered by thecompressor unit. The high-pressure gas also passes through an in-linegas filter on the system supply line to the cryocooler mounted in thedetecting unit.

A pressure gauge mounted on the compressor indicates the returnrefrigerant pressure when the system is operating. The gauge alsoindicates equalization of pressure when the system is not running. Apressure relief valve is also preferably provided in the compressorhousing to prevent operation at an unsafe pressure. A condenser isprovided to remove the heat generated during compression. Because thesystem need only provide capacity for cooling the system from ambienttemperature and then maintain operating temperatures, the compressorneed not be extremely large. In practice, operating temperatures arereached in about three hours.

The cryocooler is subject to vibration from a number of sources. First,the refrigerant compressor produces a pulsatile pressure wave in therefrigerant streams, and also transmits vibration through therefrigerant supply and return lines. At the expansion chamber, the fluidrefrigerant expands supersonically and turbulently, creating a noise andvibration. Further, the refrigerant vaporizes in the counterflow heatexchanger, creating the further possibility of vibration. The vibrationsproduced by an uncompensated cryocooler system exceed those produced bya liquid nitrogen Dewar cooling system. The uncompensated detector isvibration sensitive, resulting in an apparent loss of resolution.Therefore, the present invention employs methods for compensating forvibrations, i.e., attenuating vibration input to the detector oradjusting the detector output to eliminate the effects of vibration.

The cryocooler is physically connected to an internal mass damper with aplurality prestressed flexed copper members. These prestressed coppermembers provide a thermal communication path as well as mechanicalsupport. The cryocooler is mounted to the housing. The internal massdamper is thus cooled by the cryocooler, and has a significant thermalinertia. Therefore, it is desired to have an internal mass damper with ahigh mass-to-thermal capacity ratio for faster responsiveness. Theinternal mass damper is further thermally linked to the cold finger by aflexed high-flexibility copper strap system having a number of straps ofvarying placement and orientation. The cold finger is mechanicallysupported in the housing by a separate low thermal conductivity memberformed as a star of G-10 fiberglass reinforced epoxy material.

Thus, vibrations due to pulsation in the refrigerant flow, turbulence ormechanical vibrations transmitted in and along the refrigerant supplyand return tubes are filtered from the detector. The inertial massdamper and cold finger assembly are vibration-isolated from the housing.

The evacuated space in the detecting unit is initially evacuated to apressure of approximately 1₋₋ 10⁻⁷ Torr. This vacuum provides a cleanenvironment for the detector crystal and FET and reduces heat convectionto the cold finger by gas molecules. The operating parameters allow useof the detector until the internal pressure reaches about 1₋₋ 10⁻³ Torr,therefore providing a margin for vacuum degradation.

The present invention also provides an improved system for maintainingthe functional integrity of the vacuum in the evacuated space, improvingthe ultimate performance of systems by achieving lower temperatures andby reducing electronic and mechanical noise characteristics.

The detecting unit is sealed at the factory, and does not require anyexternal vacuum pumping during operation. It thus remains isolated fromthe external atmosphere, reducing the possibility of contamination ofthe detector or instrument. Therefore, according to one embodiment ofthe present invention, in order to maintain a high level of vacuuminside the detecting unit after sealing, a sufficient molecular sorptiongetter material is attached to a cold surface of the device to maintainmolecular flow of gas molecules, e.g., a pressure of less than about 1mTorr. Molecular sorption getter materials have the characteristic that,as cooled, they increasingly trap gasses from the cryostat, thus actingas an internal vacuum pump for the detecting unit. The total amount ofgas that can be trapped by the getter material increases as the materialgets colder and decreases as the ambient pressure drops. In such asystem, the amount of clean getter material, its temperature and theamount of residual gas in the cryostat determine the operating pressureof the detecting unit.

In a closed cycle gas expansion refrigeration system, with a givenrefrigerant, mass flow rate and pressure drop in the expansion chamber,the final steady state temperature depends on the heat input to thesystem, which includes parasitic heat input as well as heat generated byany active components. During cool-down of the cryostat, the mass flowrate of the refrigerant is reduced by reducing the throttle aperture tothe expansion chamber as the temperature drops. This throttle valve ispreferably a temperature-responsive needle valve. This reduction in flowrate increases the pressure and temperature drop of the fluid across theaperture as the temperature decreases. This reduction in refrigerantflow rate, however, also reduces the heat capacity of the system. Inorder for the system to continue cooling, the total heat input to thesystem must be lower than the heat capacity of the cooler at the currenttemperature.

When operating at steady state, the heat capacity of the cryogeniccooling system is balanced with the heat input to the system. Therefore,the system is designed to reach a target temperature based on the heatof operation of the device and the parasitic heat input to the system.Therefore, the system preferably has a low and constant parasitic heatinput so that the device operates stably over extended periods of time.

It is noted that hybrid cooling systems may be provided, combiningJoule-Thomson. Peltier, volatilizing gas, Stirling cycle cooling andother known cooling methods.

The getter material in the detecting unit must be clean enough and ofsufficient quantity to assure that the conducted heat input through theresidual gas, in combination with other sources of heat, is always lowerthan the heat capacity of the refrigerant unit. This holds true untilthe cryostat pressures of less than about 1₋₋ 10⁻³ Torr are achieved.This pressure marks the delineation between viscous and molecular gasflow and is the practical limit at which heat conduction through theresidual gas is significant. An appropriate amount of clean gettermaterial will allow the unit to achieve vacuum levels of less than about1₋₋ 10⁻³ Torr quickly, assuring that the cryocooler will achieve thedesired operating temperature range.

The molecular sorption getter is preferably formed to occupy void spaceadjacent to cooled surfaces inside the evacuated space. However, thegetter may also be provided as a plurality small portions mechanicallyretained against the cooled surfaces.

Activated carbon is a preferred getter material, and may be generatedfrom preformed organic material which is converted to activated carbonwhile retaining its form. Thus, an easily formed organic material suchas a polymer or biological material is pyrolyzed or processed intoactivated carbon which less easily shaped.

According to a further embodiment according to the present invention, anelectrically insulating gas conduit is provided in the gas supply andreturn lines to insulate the ground of the detecting unit electronicsfrom the ground of the cryocooler compressor. Proper operation of thedetector requires isolation of the detector electronics from thecompressor system.

Failure to isolate the detector results in statistical noise, much asvibration results in statistical noise, resulting in reduced low levelsensitivity and increased peak width, important qualities of thedetector. See, Quantitative X-ray Spectrometry, Jenkins et al., MarcelDekker, Inc., pp. 144-147; Scanning Electron Microscopy and X-RayMicroanalysis, Goldstein et al., (2nd Ed.) Plenum Press (1992), pp.310-393.

According to standard practice, flexible conductive metal gas supplylines are employed to contain the pressure and ensure leak freeperformance and impermeability to refrigerant gas and water. Accordingto the present invention, a glass or ceramic, or other type of pressurestable high pressure containment conduit, formed as a short tube, isplaced in line with the supply and return gas line, thus electricallyisolating the detector while containing the refrigerant gas. A preferredmaterial is Macor®, a mica filled composition.

Various interfering factors, such as vibration and electricalinterference, act to increase the FWHM of the output signal from thepreamplifier FET from less than or equal to about 137 eV at 5.9 keV from⁵⁵ Fe at 1000 cps with a 40 μS time constant to a greater value, e.g.,greater than about 140 eV. The interfering factors convolve with thesignal produced by an X-ray to increase the FWHM. See, Goldstein, J. I.,et al., Scanning Electron Microscopy and X-Ray Microanalysis (2d Ed.),Plenum Press, New York (1992), pp. 310-313. As the peak width increases,the ability to distinguish closely spaced peaks diminishes.

There are a number of methods available for reducing the effectivevibration sensitivity of the detector other than passively damping thevibration. First, a compensating detector crystal subject to nearlyidentical vibrational variation, and not subject to an unknown X-rayradiation may be used in a differential amplifier circuit. Second, anactive damping system may be provided which uses actuators, e.g.,piezoelectric actuators in proximity to the cold finger or detector, toactively suppress vibration at the detector, based on a measuredvibration pattern. Third, an accelerometer may be provided, such as amicromachined sensor, which measures the acceleration forces to whichthe detector is subject. The accelerometer output is then used tocompensate the detector output. Other types of vibration sensors mayalso be employed.

It is believed that one component of vibration sensitivity is related toa relative movement between mechanical structures of various highsensitivity electronic elements held at various potentials, such as theFET gate, and ground planes. The front face of the detector crystal isheld about 750 V from ground, in order to sweep charge from the detectorcrystal to the gate of the FET for measurement. Thus, vibrations mayaffect capacitance to ground and induce gain changes in the FET, leadingto loss of output resolution. Therefore, sensitivity to vibration ofthese structures may be reduced by shielding with a guard ring driven toapproximately the same potential as the sensitive structures orshielding with a shield mechanically fixed to the structure. Themovements of these charged structures may also be detectedelectrometrically for compensation of the output of the detectoramplifier.

Vibration may also be directly measured. For example, the Analog DevicesADXL05, having a minimum sensitivity of 5 mg, or similar device, may beemployed to detect vibration for compensation. Other types ofacceleration or vibration sensors may also be used, to measure thevibration of the detector or a surrogate clement which has adeterminable vibrational-relation to the detector.

The closed circuit cryogenic cooler, without damping, produces anunacceptable level of vibration, e.g., about 7.4 mg RMS, over afrequency range of 0-500 Hz, along the long axis of the cold finger.When two 27.75 lb steel blocks are placed on the refrigerant tubesbetween the compressor and the cryocooler, the level is reduced to about6.5 mg RMS. When a 13 lb lead ring is placed around the housing of thecryocooler, the vibration levels are reduced to about 4.5 mg RMS. Whenboth the steel block and the lead ring are employed, the vibrationlevels are reduced to about 3.2 mg RMS.

As constructed into a cryogenic radiation detector according to apreferred embodiment of the invention, with external mass damping of therefrigerant supply and return lines and internal inertial mass damping,the system produces 3.5 mg RMS from 0-2 kHz along the X axis (longaxis), 1 mg RMS along the Y axis and 1.3 mg RMS along the Z axis. Acomparable 10 liter liquid nitrogen cooled Dewar system produces 0.4 mgRMS from 0-2 kHz along the X axis (long axis), and around 0.5 mg RMSalong the Y and Z axes. With this level of vibration damping, energyresolutions of less than or equal to 137 eV at 5.9 keV from an ⁵⁵ Fesource, 1000 cps with a 40 μS time constant can be obtained with alithium drifted silicon crystal detector according to the presentinvention. Without such damping, the energy resolution would be atminimum about 140 eV, and likely greater than about 150 eV under thesame conditions, which results in an inferior instrument.

As shown in FIG. 1, three X-ray peaks 61, 62, 63 are emitted from asample. Due to characteristics of the detector system, vibration,electrical interference, and other sources of noise, the energies asreceived are dispersed to a received signal having a greater FWHM 64,65, 66. The detector, because of the closeness of these peaks, receivesa signal which appears like signal 67, and has difficulty resolving theindividual peaks 64, 65, 66. Further, as the peaks are spread, theintensity is decreased, and the signal-to-noise ratio decreased. TheX-ray peaks 61, 62, 63 emitted from a sample have a FWHM of about 2 to 3eV. The detector and associated electronic circuit, however, spreads theenergy over a wider range. For example, a liquid nitrogen cooled Dewarsystem having a lithium drifted silicon crystal operating at about100-110K has a peak width of about 137 eV using a standard measurementtechnique, measurement of a 5.9 keV X-ray from an ⁵⁵ Fe source, 1000 cpswith a 40 μS time constant (Si(Li) crystal detector). As stated above,this peak is broadened by vibration, high temperatures, and otherfactors known in the art.

In order to reduce the effects of external disturbances on the X-raydetector crystal, three general methods are theoretically available.First, the external disturbances may be filtered, attenuated, shieldedor other wise blocked. Second, the sensor may be made intrinsicallyselective or more selective for the environmental variable of interest,e.g., X-ray radiation. Finally, the sensor output may be compensated forthe effects of environmental variables other than the variable ofinterest. The second method is the province of sensor design proper, andis not the subject of the present invention, except insofar as theselectivity of the sensor is interrelated with the filtering ofinterfering environmental influences.

In the present case, the external influence of most concern isvibration. This vibration is derived from two general sources: thecompressor, which has reciprocating elements which produce a mechanicalvibration as well as a pressure pulsation in the refrigerant lines, andthe cryocooler, which is subject to turbulent flow and expansion ofrefrigerant. In order to reduce mechanical vibration from thecompressor, the compressor may be dynamically balanced and otherwiseisolated and mounted to reduce vibration which is transmitted along thesupply tubing or the refrigerant itself. Pressure pulsations in therefrigerant tubing may be filtered with resonators, bleed valves,pressure relief valves, compliant conduits and other known methods.

As stated above, vibrational energy which remains, may be damped byproviding a large mass damper firmly linked to the refrigerant tubing.Vibrations passing the linkage are attenuated. The turbulent vibrationis generated in the cryocooler proper, and therefore may not be easilyfiltered in the refrigerant lines. In order to attenuate vibrationsgenerated in the cryocooler, an active or passive mechanical filter isprovided at the cryocooler or between the cryocooler and the detectorcrystal. Passive filters include damping elements, which convertvibrations to other forms of energy. Active filters determine a force ofa vibrational wave and apply an opposing force which sums with thevibrational wave, resulting in an attenuated waveform, at least in aparticular region of interest, i.e., the detector. Since a complexmechanical structure bridges the cryocooler to the X-ray detectorcrystal, care may be taken to reduce undesired resonances and otherdetrimental acoustic or vibrational properties of the structure.

Depending on the wave source and its coupling to the mechanicalstructure, the vibrational wave may have compression wave and shear wavecomponents. These components are dispersive, and may be filteredseparately in terms of eliminating their effect on the X-ray sensor.

According to one embodiment, a thermally conductive viscous fluid isprovided in a compliant conduit as a link between the cryocooler and thedetector. This fluid may be, e.g., a vacuum oil, such as a lightsilicone oil. This fluid should have a viscosity such that shear wavesare substantially attenuated by the fluid. The compliant conduit is suchthat the conduit wall itself does not substantially support significantvibration transmission and the compliance is sufficient to attenuatecompressive waves in the fluid. The fluid is thermally conductive sothat the detector is effectively cooled. The fluid may also support aconvection current to reduce the thermal gradient along the conduit, inthe manner of a heat pipe. When a heat pipe configuration is employed, aphase change fluid medium may be employed, which has a higher vaporpressure at an end proximal to the detector than at the other endproximal to the cryocooler.

While it is preferred that the amount of vibrational energy that reachesthe X-ray detector crystal be minimized, if an output compensationsystem is also employed, it is also desired that any mechanicalhysteresis effects be minimized and sources thereof eliminated between apoint where a vibrational measurement may be obtained and the detector,even if this results in a somewhat greater amplitude vibration at thedetector crystal. This is because an electronic vibration compensationsystem requires a well characterized and repeatable effect of measuredvariables on the output of the sensor, which is not possible withhysteresis effects present. Thus, for example, where a vibrationaldetector is mounted external to the evacuated chamber, and the vibrationat the housing is presumed to correlate with the vibration at thedetector crystal, all intervening sources of hysteresis should beminimized. Where a vibrational detector detects a vibration in the coldfinger, damping systems which allow hysteresis located between thevibrational detector and the cryocooler are permissible.

For example, an optical interferometry technique may be employed, usinga source of polarized, coherent light, which is transmitted from a knownposition such as a point on the inside of the evacuated housing, throughthe evacuated space, and incident on a portion of the vibratingassembly, which is reflected to a receiving system to measure adisplacement of the portion based on an interference pattern, therebyindicating the second integral of the vibrational force. The opticalpaths may include fiber optics. Since this measured portion of theinterferometer system may be placed close to or on the detector crystal,the mechanical limitations on the support structure are minimized.Multiple optical paths may be provided to compensate for differentdegrees of freedom.

Likewise, a magnet, e.g., a high temperature superconductor magnet, maybe mounted in close proximity to the detector crystal, and an electricalcircuit provided to induce a magnetic field. A Hall effect transducer orother type magnetometer mounted on the housing in proximity to themagnet will produce a waveform which corresponds to the movement of themagnet with respect to the Hall effect transducer. High temperaturesuperconductors may also be used as part of a magnetic bearing system toisolate the detector from vibration. Therefore, the cryogenictemperatures near the detector may advantageously be used to cool hightemperature superconductors, useful as a part of sensors, actuators andisolators.

Another source of the vibration sensitivity of the X-ray detectorcrystal is the variation in distance or pressure applied by the contactbetween the FET gate electrode and the rear face of the detectorcrystal. Thus, this force may be detected directly by a transducer onthis contact, such as a strain gage, or by mounting a relativedisplacement sensor between the FET and the X-ray detector crystal. Sucha relative displacement detector may also be an interferometric oroptical, magnetic, inductive, resistive, piezoelectric, or other knowntypes. It is preferred that the only contact to the detector crystal bethe contact for the FET, which is preferably a flexed beryllium copperspring contact, so that the vibration measuring system should be of anon-contact design or measure the contact itself. A vibrational sensormay also be placed on or near the detector crystal.

In order to output compensate the detector crystal, the relation of avibration to an alteration of the output of the crystal is determined.After determining the effect of vibration on the detector, it is onlynecessary to determine the vibration at the detector to compensate theoutput. This may be explicitly measured, or determined empirically.Thus, a particular force or force waveform may be determined to have aparticular effect on the output, so that by measuring the vibrationalforce, a direct output correction may be made. Alternatively, a neuralnetwork may be trained using one or more radiation standards, whichallows the relationship between a detected vibration and the output tobe determined and compensated, based on a series of stored coefficientsand a related topology. In such output compensation systems, thecompensation need not be limited to vibration only, and in fact anymeasured variable may be compensated in like manner. Further, anynon-linearities in the sensors may advantageously be compensated by aneural network compensation scheme.

The compensation system may have a high computational complexity. Forthis purpose, a digital signal processor or dedicated neural networkprocessor may be employed to completely or partially process thetransducer signals. Where the mechanical system is complex, such aswhere vibrational transducers are mounted on the housing, and thevibration of many mechanical elements must be compensated, amathematical model of the system may be formulated to allowdetermination of a relevant vibration of the sensitive structure. Aseparate compensation may then be applied using the measured detectoroutput and the determined vibration to produce a compensated output.Where this model is fully mathematically determined, it may bepreferable to employ a mathematical processor, such as a digital signalprocessor or floating point processor to calculate the model equations.On the other hand, where the model is not mathematically determined, aneural network processor may be used to infer a vibration of the crystalbased on measured vibrations. A parallel processing system, e.g.,parallel Intel Pentium processors, Power PC, TI 32C080 (MVP) or otherRISC, CISC or DSP processors, may also be employed.

Another factor that may increase the signal spreading of the sensor iselectrical currents through the structure. These may be related tovarying electrical or magnetic fields, ground loops, leakage or theinfluence of other devices. These signal spreading influences may bereduced by electrically insulating the sensor system from externalsystems. In addition, external electrical signals may be measured andcompensated.

In order to insulate the sensor system, insulating conduits may beprovided to isolate the compressor system from the sensor system. Aninsulating conduit is joined to conductive metal tubing from thecompressor, which is, e.g., copper tubing, by stainless steel couplingswhich are connected to the insulating material at a leak-tight brazedjoint. A metal collar, which may be stainless steel, is brazed to theinsulating conduit with a standard brazing compound, to form a tightseal and a firm mechanical connection. The stainless steel couplings areheld by the brazed collars, and allow connection to the supply andreturn tubing in standard manner on both sides. A quick-release couplingof standard type, e.g., available from Aeroquip, may be providedintegral to the insulating conduit apparatus, or as a separateconnector. The preferred insulating conduit is formed of Macor®, amica-filled ceramic-type material having good fabrication properties.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features in accordance with the invention areillustrated, by way of example, in specific embodiments of the inventionnow to be described with reference to the accompanying drawings, inwhich:

FIG. 1 is a graph showing the influence of peak spreading on thedetector performance;

FIG. 2 is a block diagram of a closed circuit cooler employed forachieving cryogenic temperatures;

FIG. 3a shows a front view of the compressor;

FIG. 3b is a perspective view of a closed circuit cryogenic cooleraccording to the present invention having an external damping mass;

FIG. 4 is a cross-sectional view of an X-ray radiation detector inaccordance with a first embodiment according to the present invention;

FIG. 5 is a cross-sectional view of an X-ray radiation detector inaccordance with a second embodiment of the present invention, havingadded getter material;

FIG. 6 is a cross sectional view of an X-ray radiation detector inaccordance with a third embodiment of the present invention, having acold tube circumjacent the cold finger;

FIG. 7 is a cutaway view of the X-ray radiation detector of FIG. 6 alongline A--A, showing a mechanical support for the cold tube;

FIG. 8 is a cross section of an insulating tube insert according to thepresent invention;

FIG. 9 is a perspective view of an X-ray detector having an externalvibration sensor; and

FIG. 10 is a side view of an optical interferometer for measuringvibrational-induced displacements of the detector crystal.

It should be noted that these Figures are not drawn to scale, and therelative dimensions and proportions of some pans have been greatlyexaggerated or reduced for the sake of clarity and convenience in thedrawing. Furthermore, some pans of the cryogenic radiation detectorwhich it are not necessary to describe for an understanding of how toperform the present invention have not been shown in the drawings, butmay be provided in known manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

The operation and use of the device of Example 1, shown in FIGS. 2, 3and 5 is described in "CryoX (TM) Service Manual", EDAX, Rev. 1, March1995, attached hereto as an appendix and incorporated herein byreference.

In FIG. 9 cryogenic cooling system gas supply having a compressor 101, aheat exchanger 103 for cooling the compressed gas, also known as acondenser are provided. The compressor is electrically powered,preferably from line current through a line cord 47, shown in FIG. 3.The heat exchanger 103 is provided with a high surface area radiatorportion which has a high heat transfer coefficient between an internalvolume of compressed gas and a stream of air 48 forced over the portionby blowers 104.

The cryogenic cooling system has a supply tube 1, from a coupling 111,which feeds a stream of compressed refrigerant at approximately ambienttemperature. A return tube 117 is provided to recycle the gas throughthe system from the cryocooler 120.

The heat exchanger 103 may be a gas-to-fluid type, and the cryogeniccooling system may be an open circuit or operate off a tank ofcompressed refrigerant gas with a pressure regulator. Secondary coolingsystems may also be employed. Other types of compressed gas supplies areknown, and may be used in accordance with the teachings of theinvention.

A pressure relief valve 121 is provided in the supply line to preventoverpressure conditions. A supply line gas filter 109 and a return linegas filter 122 is are provided to reduce particulate contamination. Apressure gauge 118 is provided to indicate line equalization, and apressure relief valve 116 is also provided in the return line proximateto the cryocooler.

The compressor employs an oil lubricant, and thus includes an oilseparator 106, and oil return line 107.

As shown diagrammatically in FIG. 2, the cryocooler operates on astandard Joule-Thomson principal, with a counterflow heat exchanger forprecooling the compressed refrigerant in the supply path 112 with theexpanded refrigerant in the return path 120. The compressed refrigerantis present in a mostly liquified state at the end of the counterflowheat exchanger. A throttle valve 113 is provided to limit refrigerantflow into the expansion chamber 114, where the pressure drop in the gasflowing through the orifice occurs supersonically and isenthalpically sothat the refrigerant experiences a temperature reduction, in accordancewith the Joule-Thomson principle. The refrigerant flow in the expansionchamber is turbulent.

An external mass damper 46, formed of a steel block having a weightsufficient to damp compressor induced vibrations, e.g., greater thanabout 20 lbs., is provided firmly connected to the supply 43 and return44 lines and resting on the floor to reduce any pulsatile variation inthe refrigerant tubes.

Other types of refrigerant line vibration damping may be provided inknown manner, e.g., to provide a constant pressure supply with lowripple. A further pressure fluctuation filtering may be effected byselectively venting a portion of the compressed refrigerant gas supplyto the return line, in order to effect pressure fluctuation filtering ofthe supply line. A resonator may also be employed to damp vibration ofpredetermined frequencies.

The output of the supply line from the external mass damper 1 leads tothe cryogenic refrigerator of the radiation detector device. As statedabove, the cryogenic refrigerator includes a counterflow heat exchanger3 which precools the compressed supply refrigerant to an almost liquidstate with the returning expanded refrigerant. The expanded refrigerantvaporizes in the counterflow heat exchanger 3. The precooled pressurizedsupply refrigerant passes through a throttle valve 113, whichselectively restricts the flow of refrigerant into an expansion chamber114. The throttle 113 valve includes a needle, having a portion whichextends into the expansion chamber, with an expanded neck portion whichis proximate to a conical valve seat (not shown). As the expansionchamber 114 cools, the needle contracts, causing the expanded neck torestrict refrigerant flow by partially seating in the conical valveseat. In general, the throttle valve 113 is more open during cool down,where the cryogenic cooling system has a greater cooling capacity, butreaches a higher minimum temperature because of the increased backpressure in the expansion chamber. After the system reaches a targettemperature, the throttle valve 113 is less open, so that the flow rateis reduced and the pressure differential between the compressedrefrigerant supply 112 and the expansion chamber 114 is increased. Thisdecreases the theoretical minimum temperature while having the effect ofreducing the heat removal capacity of the system. This type of controlhelps to achieve cool down rapidly, while providing integral temperatureregulation.

As shown in FIG. 4, The cryogenic refrigerator and the cooled componentsof the detector are contained within an evacuated chamber 15, 20, 21,25. The vacuum reduces the thermal conduction through a surrounding gasto the outside, which is, e.g., at ambient temperature. The evacuatedchamber 15, 20, 21, 25 is initially brought, during manufacture, to avacuum pressure of about 1₋₋ 10⁻⁷ Torr. This allows an amount ofdegradation of the vacuum, to about 1₋₋ 10⁻³ Torr (operating), beforethe thermal transfer through the gas in the envelope becomes limiting,and the gas tends to be less molecular and more viscous in itscharacteristics. This degradation of the vacuum may occur due to, e.g.,outgassing of the detector or diffusion through seals. Since theevacuated chamber is scaled, degradation of the vacuum limits theoperating life of the detector before repair or replacement. The housingis sealed using a single hard seal, available from Helicoflex (notshown).

Therefore, low outgassing components are employed in the construction ofthe device. Components are baked under vacuum during final assembly atleast 150 F for at least 24 hours to eliminate as much residual gas aspossible and to minimize later outgassing. The detector crystal 34 andelectronics 28, including the FET, cannot be baked at high temperatures,and thus are a major source of outgassing in the final assembly.

The expansion chamber 114 of the cryogenic refrigerator is linked inthermal communication to an internal mass damper 8, provided in order toreduce vibration transmitted to the radiation detector 34. This internalmass damper 8 reduces vibration transmitted through the refrigerantsupply 1 and return 2 lines, as well as any vibration from turbulence,resonance or other vibration from the expansion chamber 4. The internalmass damper 8 may also reduce mechanical coupling from externalvibration in the supply 1 and return 2 lines to the detector system. Theinternal mass damper 8 is a copper cylinder weighing about 2 lbs.Vibrations reduce the resolution of the detector 34 and may createmicrophonics in the output signal.

The cryocooler 3 is mechanically and thermally linked to the internalmass damper 8 by a plurality prestressed flexible copper straps 6, shownin FIG. 6, which are flexed in position. This connection serves to allowthe internal mass damper 8 to attenuate vibrations from the cryocooler3, while acting as a thermal conduit. These preflexed flexible copperstraps 6 also serve to mechanically support the internal mass damper 8.

The internal mass damper 8 is linked to a flexible thermal coupling 10to the cold finger 32. This flexible thermal coupling 10 consists of aplurality of flexible braided copper webs, which are flexed to providemaximum compliance along the axis of the cold finger 32. The cold finger32 is thus supported separately from the cryocooler 3, as the flexiblethermal coupling 10 between the internal inertial mass damper 8 and thecold finger 32 does not provide stiff mechanical support. The flexiblethermal coupling 10, however, cannot be formed with sufficientflexibility to provide sufficient vibration isolation, and therefore italone is insufficient to vibration isolate the detector 34.

The cold finger 32, which is preferably formed of copper, isconcentrically secured within an extension of the envelope 35 from thecryocooler 3 and internal mass damper 8 by a thermally insulating disk17. The thermally insulating disk 17 preferably is formed of G-10fiberglass reinforced epoxy. Near the detector, a further spacingsupport is provided as a strip of Velcro® fastener hook portion 26 about0.25 inch wide, wrapped circumferentially around the detector holder 23,which centers the detector holder 23 in the extension of the envelope35.

Other materials, e.g., a fluorocarbon plastic commercially available asa polychlorotrifluoroethylene or PCTFE, (Kel-f®) may also be used. Thethermally insulating disk 17 for the cold finger has radially directedmembers in a star formation to reduce the heat conduction path crosssectional area. The cold finger 32 may be supported, if necessary, by anumber of insulating disks 17, spaced along the length of the coldfinger 32.

The cold finger 32 is a circular cylindrical copper cylinder which ispreferably about 0.250 inches in outside diameter and is at atemperature of less than about 100K with a temperature gradient from endto end. The cold finger 32 has a polished external surface that has aspecular finish to provide a relatively low emissivity factor, forexample, around 0.1. This low emissivity minimizes radiation coupling ofthe cold finger 32 to the housing members 35, 21, 15, which radiationcoupling is the major source of heat transfer. The cold finger 32 isthen wrapped in aluminized Mylar® sheet.

The cryogenically cooled detector assembly is part of an X-rayspectrometer, which may be mounted to an electron microscope chamber.The microscope chamber may be evacuated, drawn to, for example, 1₋₋ 10⁻⁶to 1₋₋ 10⁻⁵ Torr. A specimen is located within a microscope chamber andis located in the path of an electron beam of the microscope. Anelectron beam is incident on the specimen and emits X-rays of particularenergies at particular angles. These radiated X-rays are received by thedetector assembly. The detector may be moved with respect to the sample,to collect data relating to X-ray emission at differing orientations.

A radiation detector holder 23 assembly is thermally conductivelysecured to the cold finger 32. This radiation detector holder 23 ispreferably formed of aluminum. The extension of the envelope 35 has athin, X-ray radiation transparent window 25 formed at an end thereof.This window 25 is sealed, so that the vacuum within the housing ismaintained.

The radiation detector holder 23 assembly holds an X-ray detectorcrystal 34, which is preferably a lithium drifted silicon X-raydetector, as known in the art. The preferred size is 10 mm². Thiscrystal detector 34 operates with an externally generated bias voltage,to sweep the induced charge to the gate of a FET 28. A wire conductorcouples the sleeve to a source of bias voltage (not shown) for biasingdetector. The FET is electrically and mechanically coupled to thedetector crystal by a beryllium-copper spring 36, which resilientlyholds the detector crystal 34 in place. The electrical output from theFET amplifier circuit is a thin wire which exits from the envelope forconnection to other electronic circuits (not shown).

The radiation detector holder 23 assembly also supports other portionsof a field effect transistor (FET) electrical amplification circuit (notshown). Other elements of the electronic circuit include a resistiveheater, which dissipates about 0.25 W, provided to heat the FET by about40K, several diodes, a light emitting diode to blank the FET, and otherelements. If other electrically dissipative elements are provided, theymay advantageously be mounted near the FET to help provide the necessaryheat.

Other types of detectors are known, and may be employed, includinggermanium crystals, used as X-ray or gamma ray detectors, and whichgenerally require lower temperatures for high performance than silicon(lithium drifted) crystal detectors.

EXAMPLE 2

The apparatus generally according to Example 1 is provided with a largeamount molecular sorption getter 5, 16, provided as portions ofactivated carbon material, as shown in FIGS. 5 and 6. According to thisembodiment, the void volume 22 in the evacuated chamber is minimized.This is in contrast to the design according to Example 1, wherein thevoid volume 22 is not particularly minimized, except to reduce theinternal surface area. In fact, under normal circumstances, the voidvolume in the apparatus of Example 1 is not minimized, so that theadverse effects of outgassing are reduced or diluted in a larger volume.

The molecular sorption getter 5, 16 therefore may occupy a significantportion of the void volume 22 in the evacuated space. For example, themolecular sorption getter 5, 16 may be placed around the expansionchamber 4 and adjacent to the counter flow heat exchanger 3. Themolecular sorption getter 5 preferably does not link the internal massdamper 8 and the cryocooler 3, as these should be free for effectivedamping of vibration.

An additional molecular sorption getter 16 may also be provided on theinternal mass damper 3, e.g., adjacent to the flexible copper thermallyconductive strap 10.

The molecular sorption getter 5 is provided in firm thermal contact withcooled components of the device, by, e.g., a thermally conductive epoxy.Such a thermally conductive epoxy may be a silver powder filled two-partepoxy material. Additional molecular sorption getter 16 material isprovided in a thermally conductive pouch mounted with epoxy to theinternal inertial mass damper 8, e.g., thin wall aluminum foil havinggas permeable apertures.

Alternatively, the molecular sorption getter may be provided as anorganic polymer shaped in the desired configuration which is pyrolyzedto produce a shaped getter. The molecular sorption getter may also be azeolite or synthetic zeolite material, as known in the art.

Molecular sorption getter materials have the characteristic that theiraffinity for molecular species increases as the temperature decreases.Thus, as the cryogenic detector device is initially cooled, the pressurein the void volume 22 decreases, as the number of free gas molecules inthe evacuated space is reduced. This reduces the heat conduction, thusreducing the parasitic heat load on the cryogenic refrigerator system.

EXAMPLE 3

As shown in FIGS. 6 and 7, a cold sleeve system similar to that employedin U.S. Pat. No. 5,235,817, incorporated herein by reference, may beused to allow the present cryocooler to achieve the temperaturesnecessary for high performance X-ray detectors, e.g., less than about95K. As shown in FIGS. 6 and 7, a cold sleeve 26 is providedconcentrically around the cold finger 32, with the cold sleeve 26thermally connected to the cryocooler 3 through a separate strap 13.Since the cold sleeve 26 is isolated from the cold finger 32 and thedetector 34, the cold sleeve need not be fully vibration isolated. Thiscooled sleeve 26, a tube circumjacent to the cold finger 32, shuntsparasitic radiated heat input away from the cold finger 32 structure ina separate heat path to the cryocooler. Further, the thermal radiationbetween surfaces is minimized by providing specular surfaces and byminimizing the surface areas of the facing surfaces. The closeness ofspacing of the cold finger 32 and cold sleeve 26 is inconsequential withrespect to the radiation coupling because of the low attenuation throughthe vacuum of the void volume 22. In this case, the cold sleeve 26 maybe a copper tube 7/16" in inner diameter. At a given temperature, theamount of radiation from or to a member will be proportional to itssurface area, and thus the thermal coupling between two concentric tubeswill be minimized by reducing their surface areas.

The cold sleeve is supported and centered within the extension of thehousing 35 by a support disk 90.

Advantageously, the cold sleeve is provided with an aperture 37 exposingthe FET 28 to the radiant heat from the warm extension of the housing35. This aperture 37 reduces the need for heating of the FET 28 toprovide optimum temperature for operation. Further, the aperture 37 alsoallows the cold finger 32 to be centered and supported by a supportmember 29, which passes through the aperture. The support member 29provides a further heat flow path to warm the FET 28. Thus, the coldsleeve 26 allows reduced parasitic heat input to the cold finger 32,thus allowing a smaller temperature gradient along its length.

EXAMPLE 4

As shown in FIG. 9, the apparatus generally according to Examples 1, 2or 3, may be provided with a vibration sensor 100, in order toelectrically compensate for vibration-induced output variations. Inorder to allow accurate compensation, it is preferred that hysteresis beeliminated as much as possible and that the device output be repeatable.Thus, for example, adhesives, such as might be used to attach Velcro,have creep, and contribute the hysteresis. Loosely packed gettergranules may shift due to vibration, and are avoided. Mylar sheet mustbe wrapped firmly and affixed securely. Other components should also bedesigned and mounted to eliminate hysteresis.

It should be noted that, with output compensation, mechanical dampingmay not be necessary.

A three axis vibration sensor 100 having a 25-100 mg full scale range,is mounted on the outer casing 15 of the cryocooler. The vibrationsensor 100 output is digitized with a 16 bit resolution analog todigital converter at between about 10-50 kHz sample rate. The dataacquisition system is an ISA card in an Intel Pentium 90 MHzprocessor-based desktop computer 101. The processor first processes thevibration sensor data alone in order to determine calculate a vibrationforce at the detector crystal. The processor then calculates acompensation of the detector crystal output based on the measuredvibration.

A model of the vibration is generated by instrumenting the sensor systemto actually measure vibration at the detector crystal position, whilerunning the cryocooler, and measuring vibration with the externalvibration sensor 100. This instrumented setup correlates measuredvibration by the external vibration sensor with vibration at thedetector crystal. The effect of vibration on the detector crystal isthen determined by operating the detector system using standardizedradiation sources, thereby allowing determination of the effect ofvibration on the sensor output.

Acceptable types of vibration sensors include piezoelectric sensors,strain gages, moving mass accelerometers, optical, fiber optic, andother known types.

The sensor system may also be compensated for other factors, such asslight variations in electron beam voltage or current, temperaturevariations, vacuum pressure, time since crystal detector blanking, timesince FET blanking, and other factors.

EXAMPLE 5

As shown in FIG. 10, the system generally according to Example 4 isprovided, with the exception that a fiber optic interferometer isprovided to determine a relative motion between the detector holder 23and the housing 35. A reflective portion 103 is provided on an innersurface of the housing 35. A laser diode 105, producing a coherentinfrared polarized monochromatic light, e.g., at about 940 μm, iscoupled to an optical fiber 102, which terminates near the FET 28 andtransmits the light. Light exits the optical fiber 102 and is directedthrough a semireflective mirror 106 to the reflective portion 103. Thedetector crystal 34 may be shielded from the illumination to preventdetector artifacts. A portion of the light exiting the fiber is alsodirected by the semireflective mirror 106 directly to a photodetector104. Light reflecting off the reflective portion 103 is also reflectedto the photodetector 104 by the semireflective mirror 106 to produce aninterference pattern relating to a difference in path length of the twolight paths, modulating the amplitude of the photodetector 104 output.The output is transmitted through a wire 106. Therefore, by detectingthe intensity of light by the photodetector 104, the slight change indistance between the tip of the optical fiber 102 detector and thehousing 35 due to vibration may be determined. The photodetector 104output is lead from the evacuated chamber and fed to the compensationprocessor.

X-ray radiation passes through the window 109 and is incident on thedetector crystal 34. Photonic-induced charge is swept from the detectorcrystal 34 to the gate of the FET 28 through a beryllium copper springcontact 36. Relative movement between the FET 28 and the housing 35 alsoinduces charge due to electrostatic effects. The interferometer systemallows submicron relative motions to be detected and quantified. Thus,the influence of vibration may be compensated by determining thevibration displacement waveform and compensating the output of the FET28 amplifier circuit based on a determined relationship betweenvibration and output of the amplifier circuit.

One or more additional interferometers may also be provided forvibration compensation.

In an alternative embodiment, the optical interferometer is used in aclosed circuit vibration suppression system, wherein an actuator isprovided along the axis of measurement of the interferometer. Anactuator, such as a piezoelectric actuator, is driven to null themeasured displacement. Therefore, relative displacement induced outputvariations will be suppressed.

EXAMPLE 6

In order to provide improved electrical performance, it is desired toelectrically isolate the compressor system from the detector. Thecompressor 45 is electrically operated, and thus has a magnetic fieldswhich may induce small currents. Further, the compressor 45 may bedistant from the detector, and therefore the possibility of significantground loops is present. Finally, by electrically isolating the detector34 from the compressor 45, the electronic outputs of the detector may bemore easily integrated with the electronics of the X-ray device withoutinterference.

Normal supply 1 and return 2 tubing for compressed refrigerant gas isformed of malleable copper tubing. This tubing 1, 2 is conductive.Generally available flexible non-conductive tubing may be unsuitable forthis purpose, as it may leak, introduce contaminants or kink.

The present invention employs standard conductive malleable metal tubingwith an electrical isolation device 50, as shown in FIG. 8, in line withthe tubing. The isolation device 50 must contain the high pressure ofthe supply line, without introducing contaminants.

The electrical isolation device includes a glass or ceramic length oftubing 52, preferably Macor®, which is about preferably about 5-10 cm inlength, although any length sufficient to provide electrical insulationmay be used. Each end of the tube 52 is counterbored with a recessedgroove 56. The tubing 52 is brazed to stainless steel fittings 51, 53which conform to the counterbored recessed grooves 56 to form agas-tight and mechanically strong seal. The brazed fittings 51, 53 arefurther coupled to stainless steel mechanical fittings 54, 55, forconnection to the supply 1 and return 2 line tubing.

In a preferred embodiment, on at least one side of the isolation device50, the stainless steel fitting is a standard-type self sealing, quickrelease fitting, available from Aeroquip, allowing the detector to beeasily separated from the compressor.

It should be understood that the preferred embodiments and examplesdescribed herein are for illustrative purposes only and are not to beconstrued as limiting the scope of the present invention, which isproperly delineated only in the appended claims.

What is claimed is:
 1. A cryogenic cooling apparatus for a radiationdetector, comprising:a Joule-Thomson closed cycle cryogenic coolerhaving a high pressure compressed refrigerant supply conduit, a flowlimiting orifice and a refrigerant return conduit, said Joule-Thomsonclosed cycle cryogenic cooler producing a vibration during operation; athermally conductive member having a first portion and a second portion,being in thermal and mechanical communication with said Joule-Thomsonclosed cycle cryogenic cooler at said first portion, and having anonunity vibration transmission function from said first portion to saidsecond portion of said thermally conductive member; and a cryogenicradiation detector, having a sensitivity to radiation and to vibrationand having an output, mounted in thermal communication with said secondportion of said thermally conductive member, said nonunity vibrationtransmission function modifying vibrations to which said detector issensitive, to reduce an effect of said vibration on said output.
 2. Thecryogenic cooling apparatus according to claim 1, further comprising acounter flow heat exchanger between said high pressure refrigerantsupply conduit and said refrigerant return conduit, allowing heatexchange between cooled returning refrigerant and said high pressurecompressed refrigerant supply.
 3. The cryogenic cooling apparatusaccording to claim 1, wherein said Joule-Thomson closed cycle cryogeniccooler reaches a minimum temperature of below about 77K.
 4. Thecryogenic cooling apparatus according to claim 1, wherein saidJoule-Thomson closed cycle cryogenic cooler produces an undampedvibration of at least about 5 mg RMS which is attenuated to less thanabout 4.5 mg RMS.
 5. The cryogenic cooling apparatus according to claim1, wherein said Joule-Thomson closed cycle cryogenic cooler has amaximum thermal capacity of about 2 Watts when a temperature of saidthermally conductive member is about 82K.
 6. The cryogenic coolingapparatus according to claim 1, further comprising a throttle valve atsaid flow limiting orifice, controlling a flow of refrigerant from saidhigh pressure supply conduit.
 7. The cryogenic cooling apparatusaccording to claim 6, wherein said throttle valve is responsive to atemperature of said refrigerant downstream of said flow limitingorifice.
 8. The cryogenic cooling apparatus according to claim 1,wherein said detector is an X-ray detector.
 9. The cryogenic coolingapparatus according to claim 1, wherein said detector is a lithiumdrifted silicon crystal X-ray detector.
 10. The cryogenic coolingapparatus according to claim 1, wherein said detector is a germaniumcrystal radiation detector.
 11. The cryogenic cooling apparatusaccording to claim 1, wherein said detector is an X-ray detector of anelectron microscope, achieving an energy resolution FWHM of less than orequal to about 137 eV at 5.9 keV, from an ⁵⁵ Fe source, at 1000 cps witha 40 μS time constant.
 12. The cryogenic cooling apparatus according toclaim 1, wherein said detector is an X-ray detector of an electronmicroscope having a spatial resolution, said vibration being attenuatedto a level which causes an insignificant error in said spatialresolution.
 13. The cryogenic cooling apparatus according to claim 1,wherein said detector is provided in an evacuated space.
 14. Thecryogenic cooling apparatus according to claim 13, wherein saidcryogenic cooler, thermally conductive member and detector are housed insaid evacuated space, said evacuated space being sealed with a singlevacuum seal.
 15. The cryogenic cooling apparatus according to claim 1,further comprising a vibration damper linked to said high pressurecompressed refrigerant supply conduit.
 16. The cryogenic coolingapparatus according to claim 15, further comprising a mass linked tosaid high pressure refrigerant supply conduit for reducing transmittedvibration from said high pressure compressed refrigerant supply to saidclosed cycle cryogenic cooler.
 17. The cryogenic cooling apparatusaccording to claim 15, wherein said vibration damper comprises a reliefvalve at said high pressure refrigerant supply conduit.
 18. Thecryogenic cooling apparatus according to claim 1, wherein said thermallyconductive member comprises a mechanical filter to attenuate transmittedvibrations.
 19. The cryogenic cooling apparatus according to claim 1,further comprising a mechanically compliant thermal coupling betweensaid thermally conductive member and said detector.
 20. The cryogeniccooling apparatus according to claim 1, wherein said detector isprovided in an evacuated space, further comprising a molecular sorptiongetter composition mounted in said evacuated space of sufficientquantity to achieve a molecular gas flow at cryogenic temperatures. 21.The cryogenic cooling apparatus according to claim 20, wherein said amolecular sorption getter composition is mounted in thermalcommunication with one or both of said cryogenic cooler and saidthermally conductive member.
 22. The cryogenic cooling apparatusaccording to claim 20, wherein said molecular sorption getter isactivated carbon.
 23. The cryogenic cooling apparatus according to claim20, wherein said molecular sorption getter is preformed as an organicpolymer material or a mass of biological material and subsequentlyconverted to activated carbon.
 24. The cryogenic cooling apparatusaccording to claim 1, further comprising an insulating conduit portionin sequence with said high pressure compressed refrigerant supplyconduit, electrically isolating said cryogenic cooling apparatus. 25.The cryogenic cooling apparatus according to claim 24, wherein saidinsulating conduit is a tube formed of a material selected from thegroup consisting of glass, ceramic and Macor.
 26. The cryogenic coolingapparatus according to claim 24, wherein said insulating conduit isbrazed in sealed manner to a connection fitting.
 27. A method ofcryogenically cooling a radiation detector, comprising the steps of:(a)providing:a closed cycle cryogenic cooler having a high pressurecompressed refrigerant supply conduit, an expansion chamber, an flowlimiting orifice from said compressed refrigerant supply conduit to saidexpansion chamber, and a refrigerant return conduit, said closed cyclecryogenic cooler being subject to the generation of vibration duringoperation; a thermally conductive member having a first portion and asecond portion, being in thermal and mechanical communication with saidclosed cycle cryogenic cooler at said first portion, and having anonunity vibration transmission function from said first portion to saidsecond portion of said thermally conductive member; and a cryogenicradiation detector, having a sensitivity to radiation and to vibrationand an output, mounted in thermal communication with said second portionof said thermally conductive member, said nonunity vibrationtransmission function modifying vibrations to which said detector issensitive, to reduce an effect of said vibration on said output; (b)providing a compressed refrigerant supply to said supply conduit; and(c) venting expanded refrigerant from said return refrigerant conduit.28. The method according to claim 27, further comprising the step ofrecycling said vented expanded refrigerant.
 29. The method according toclaim 28, further comprising the step of compressing said ventedexpanded refrigerant in a compressor.
 30. The method according to claims27, further comprising the step of precooling said compressed supplyrefrigerant in a counterflow heat exchanger provided between said highpressure refrigerant supply conduit and said refrigerant return conduit,cooled refrigerant from said expansion chamber flowing in saidrefrigerant return conduit cooling said compressed supply refrigerant.31. The method according to claim 27, further comprising the step ofattenuating vibration in said high pressure compressed refrigerantsupply conduit.
 32. The method according to claim 27, wherein saiddetector is a lithium drifted silicon X-ray detector.
 33. The methodaccording to claim 27, wherein said detector is a germanium radiationdetector.
 34. The method according to claim 27, further comprising thestep of controlling a flow of compressed refrigerant through saidorifice into said expansion chamber with a throttle valve provided atsaid orifice.
 35. The method according to claim 34, wherein said flow ofcompressed refrigerant is controlled based on a temperature in saidexpansion chamber.
 36. The method according to claim 27, furthercomprising the step of maintaining an evacuated inner portion of ahousing.
 37. The method according to claim 27, further comprising thestep of damping vibration from said cryogenic cooler transmitted to saiddetector.
 38. The method according to claim 27, further comprising thestep of attenuating transmitted vibration with a mechanically compliantthermal coupling between said thermally conductive member and saiddetector.
 39. The method according to claim 27, further comprising thesteps of detecting an X-ray in an electron microscope, and attenuatingvibration received by said detector so that an energy resolution isachieved having an FWHM of less than or equal to about 137 eV at 5.9 keVfrom ⁵⁵ Fe at 1000 cps with a 40 μS time constant.
 40. The methodaccording to claim 27, wherein said cryogenic cooler, thermallyconductive member and detector are housed in an evacuated chamber, saidchamber being sealed with a single vacuum seal.
 41. The method accordingto claim 27, further comprising the step of reducing transmittedvibration from said high pressure compressed refrigerant supply to saidclosed circuit cryogenic cooler by affixing a mass to said refrigerantsupply conduit.
 42. The method according to claim 27, wherein saidcryogenic cooler, thermally conductive member and detector arc housed inan evacuated chamber, further comprising the step of getteringcontaminants in said chamber with a getter material provided insufficient quantities to achieve a molecular gas flow at cryogenictemperatures.
 43. The method according to claim 42, further comprisingthe step of cooling a molecular sorption getter.
 44. The methodaccording to claim 43, wherein said molecular sorption getter isactivated carbon.
 45. The method according to claim 43, wherein saidmolecular sorption getter is preformed as an organic polymer or abiological material and converted to activated carbon.
 46. The methodaccording to claim 27, further comprising the step of electricallyisolating said compressed refrigerant supply from said detector bypassing the compressed refrigerant through an insulating conduit.
 47. Acryogenic cooling apparatus for a radiation detector, comprising:acryogenic cooler having a vibration of operation, said vibration ofoperation being of at least a first level; an uncompensated detector,having a sensitivity to radiation and to vibration, and an output, whichthe detector is mounted in thermal communication with said cryogeniccooler, said uncompensated detector producing an unacceptable outputresolution when subject to vibration of at least said first level andproducing an acceptable output resolution of at worst about 140 eV at5.9 keV from ⁵⁵ Fe at 1000 cps with a 40 μS time constant when subjectto vibration of at most a second level; and a vibration compensationsystem reducing an effect of the vibration on said detector output sothat, when said cryogenic cooler produces at least said first level ofthe vibration, said compensation system produces said acceptable outputresolution.
 48. The cryogenic cooling apparatus according to claim 47,wherein said vibration compensation system comprises a passivemechanical vibration attenuation structure.
 49. The cryogenic coolingapparatus according to claim 47, wherein said vibration compensationsystem comprises an active electro-mechanical vibration attenuationsystem.
 50. The cryogenic cooling apparatus according to claim 47,wherein said vibration compensation system comprises an activeelectronic compensation circuit at said output of said detector.
 51. Thecryogenic cooling apparatus according to claim 47, wherein saidvibration compensation system comprises a viscous fluid link having afluid which does not substantially support shear wave propagation in amechanically compliant conduit.
 52. The cryogenic cooling apparatusaccording to claim 47, wherein said vibration compensation systemcomprises a flexible thermally conductive link which damps transmittedvibration between said closed cycle cryogenic cooler and said detector.53. The cryogenic cooling apparatus according to claim 47, wherein saidvibration compensation system comprises one or more piezoelectricactuators.
 54. The cryogenic cooling apparatus according to claim 47,wherein said vibration compensation system comprises an electroniccircuit having a vibration transducer mounted to determine a vibrationof said cryogenic cooling apparatus.
 55. The cryogenic cooling apparatusaccording to claim 47, wherein said vibration compensation systemcomprises a transducer, an actuator and a control for driving saidactuator to null an output of said transducer.
 56. The cryogenic coolingapparatus according to claim 47, wherein said vibration compensationsystem comprises a compensation detector, having a response to saidvibration having a predetermined relation to a response of said detectorto said vibration.
 57. The cryogenic cooling apparatus according toclaim 47, wherein said vibration compensation system comprises a hightemperature superconductor.
 58. A detector system for measuring anenergy of an X-ray, comprising:(a) a lithium drifted silicon crystalX-ray detector system having an X-ray sensitivity and a vibrationsensitivity, X-rays and vibration each causing a variation in an outputcurrent of said x-ray detector system, said x-ray detector systemincluding an electrical amplifier circuit receiving said output currentand producing an amplified signal; (b) a support for said detectorsystem, said support being coupled to a device subject to a mechanicalvibration, said amplified signal of said x-ray detector system amplifierhaving a vibration-related signal component which, in the absence ofcompensation, increases a FWHM of said output to an amount greater thanabout 140 eV at 5.9 kev from ⁵⁵ Fe at 1000 cps with a 40 μS timeconstant; and (c) means for compensating said vibration-related increasein FWHM of said x-ray detector system amplifier to an amount less thanor equal to about 137 eV at 5.9 kev from ⁵⁵ Fe at 1000 cps with a 40 μStime constant.
 59. The detector system according to claim 58, whereinsaid compensating means comprises a mechanical vibration attenuationdevice coupled with said support.
 60. The detector system according toclaim 58, wherein said compensating means comprises an electrical signalprocessing system for processing said output.
 61. The detector systemaccording to claim 60, wherein said electrical signal processing systemcomprises a vibration sensor and a deconvolution processor.